UserManagerPlugin can auto import bad (empty) usernames

[lkraav]

i ran into a case where i had an empty line inside htpasswd file. this resulted in a user being listed in admin panel with all fields seemingly empty. actually the username consists of a single linefeed, which is visible in page source.

now, pressing Import All for UserManagerPlugin imports this empty user and it is very difficult to get rid of it. because none of Remove or Delete buttons are unable to remove this user, direct access to database is needed to fix it.

sqlite> delete from session where last_visit='1267208488';
sqlite> delete from session_attribute where length(sid) < 3;

UserManagerPlugin should probably check for username sanity before importing.